fix a crash when selling nothing.

build 687

src/build.h

@@ -1,3 +1,3 @@
 #define VERSION_MAJOR 3
 #define VERSION_MINOR 3
-#define VERSION_BUILD 686
+#define VERSION_BUILD 687

src/economy.c

@@ -2389,7 +2389,7 @@ static bool sell(unit * u, request ** sellorders, struct order *ord)
         }
     }
     else {
-        n = atoi((const char *)s);
+        n = s ? atoi(s) : 0;
         if (n == 0) {
             cmistake(u, ord, 27, MSG_COMMERCE);
             return false;